Perl_warner( aTHX_ WARN_SYNTAX, "Stringified reference\n" );

/* or you could make references REALLY strict ;-) */
if ( PL_hints & HINT_STRICT_REFS )
    Perl_croak( aTHX_ "Stringification of reference disallowed\n" );

##</code><code>##

/* in perl.h with the rest of the HINT_ defines */

+ #define HINT_STRICT_STRINGIFY       0x00000004

/* in sv.c as noted by xmath */

+ if ( PL_Hints & HINT_STRICT_STRINGIFY )
+    Perl_croak(aTHX_ "Stringification of reference disallowed");

# in strict.pm

    my %bitmask = (
+   stringify => 0x00000004,
    refs => 0x00000002,
    subs => 0x00000200,
    vars => 0x00000400
    );
    
    sub bits {
        my $bits = 0;
        foreach my $s (@_){ $bits |= $bitmask{$s} || 0; };
        $bits;
    }
    
    sub import {
        shift;
-       $^H |= bits(@_ ? @_ : qw(refs subs vars));
+       $^H |= bits(@_ ? @_ : qw(refs subs vars stringify));
    }
    
    sub unimport {
        shift;
-       $^H &= ~ bits(@_ ? @_ : qw(refs subs vars));
+       $^H &= ~ bits(@_ ? @_ : qw(refs subs vars stringify));
    }